1. Field of the Invention
The present invention relates to a gas barrier laminate film which is useful as a material for a package to be employed in the fields of foodstuffs and such non-foodstuffs as medicine and electronic articles, and in particular, to a gas barrier laminate film which is excellent in gas barrier properties so that it is capable of preventing oxygen or water vapor in an air atmosphere from entering into the package, thereby making it possible to inhibit the deterioration or denaturing of the contents of the package.
2. Description of the Related Art
In order to inhibit the denaturing of the contents of the package and hence to retain the functions and properties of the contents, a packaging material to be employed for packaging foodstuffs and such non-foodstuffs as medicine and electronic articles is required to be such that it is capable of preventing the influence of gas such as oxygen, water vapor and other kinds of gases which permeate through the packaging material and cause the denaturing of the contents of the package. Thus, the packaging material is required to have a sufficient degree of gas barrier properties to block the permeation of these gases.
For this reason, there have been conventionally mainly employed, as a gas barrier layer, a resinous film made of a polyvinyl alcohol/ethylene vinyl copolymer, polyvinylidene chloride, polyacrylonitrile, etc.; a coated plastic film having a resin coating; or a metal foil made of aluminum for instance or a metallized film formed of aluminum for instance.
As for specific examples of the resinous film, JP Patent No. 2556940 for example discloses a laminate film comprising a resin substrate and a covering layer which is obtained through the polycondensation, by means of sol-gel method, of a composition comprising alkoxy silane such as Si(O—CH3)4, silane coupling agent such as epoxy silane, and polyvinyl alcohol. Since this covering layer is formed through hydrogen bond, this covering layer can be easily swelled and dissolved as it is exposed to water. Therefore, under severe conditions such as boiling or retort-treatment, the gas barrier properties of this covering layer can easily deteriorat.
On the other hand, in the case of the metal foil and the metallized film, although they are excellent in gas barrier properties, they are accompanied with problems that it is impossible to visually recognize the contents of the package through a packaging material, that it is impossible to use a metal detector on the occasion of examination of the contents of the package, and that they are required to be disposed as a noncombustible substance on the occasion of disposal. Further, in the case of the gas barrier resin film and the film having a gas barrier resin coating, they are largely dependent on temperature and humidity so that they are incapable of retaining sufficient gas barrier properties. Furthermore, in the case of vinylidene chloride and polyacrylonitrile which have been employed as a gas barrier resin, they may become a source of harmful substance on the occasion of disposal or incineration.
Because of the aforementioned problems, there is proposed, in JP Patent No. 2790054 for example, a gas barrier packaging material which is formed of a laminate comprising a first layer composed of a vapor deposition layer of an inorganic compound; and a second layer constituted by a gas barrier covering layer which can be obtained from a process wherein a coating agent comprising, as major components, (1) a water-soluble polymer, and (2) I) at least one kind of metal alkoxide or metal alkoxide hydrolyzate, or II) an aqueous solution or water/alcohol mixed solution containing tin chloride is coated and thermally dried to obtain the gas barrier covering layer. This gas barrier packaging material is excellent in gas barrier properties, in water resistance and in humidity resistance, and is more or less heat resisant. However, since the second layer of the gas barrier packaging material is formed of a hydrogen bond between the metal alkoxide hydrolyzate and the water-soluble polymer having hydroxyl group, when the gas barrier packaging material is employed as a packaging material which is designed to be subjected to boiling treatment or retort sterilization treatment, the covering layer is caused to swell, raising the problem of deterioration of gas barrier properties. Even if the degree of deterioration of gas barrier properties is small, this kind of packaging material is not suited for use as a packaging material which is required to have very high gas barrier properties as in the case of the primary package for a transfusion liquid for instance.